1. Field of the Invention
The invention relates generally to field emission electron sources and, more particularly, to a field emission electron sources employing a carbon nanotube.
2. Discussion of Related Art
Carbon nanotubes (also herein referred to as CNTs) are very small tube-shaped structures essentially having a composition of a graphite sheet in a tubular form. Carbon nanotubes have interesting and potentially useful electrical and mechanical properties and offer potential for various uses in electronic devices. Carbon nanotubes also feature extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). These features tend to make carbon nanotubes ideal candidates for field emission electron sources.
Generally, a cnt field emission electron source includes a conductive base and a carbon nanotube formed on the conductive base. The carbon nanotube acts as an emitter of the field emission electron source. The methods adopted for forming the carbon nanotube on the conductive base mainly include mechanical methods and in-situ synthesis methods. One mechanical method is performed by placing a synthesized carbon nanotube on a conductive base by an Atomic force microscope (AFM) and then fixing the carbon nanotube on the conductive base via a conductive paste or other adhesive. The mechanical method is relatively easy to carry out. However, the precision and efficiency thereof are relatively low. Furthermore, the electrical connection between the conductive base and the carbon nanotube tends to be poor because of the limitations of the conductive pastes used therebetween. Thus, the field emission characteristics of the carbon nanotube are generally unsatisfactory.
One in-situ synthesis method is performed by coating metal catalysts on a conductive base and synthesizing a carbon nanotube on the conductive base directly by means of chemical vapor deposition (CVD). The in-situ synthesis method is relatively easily performed. Furthermore, the electrical connection between the conductive base and the carbon nanotube is typically good because of the direct engagement therebetween. However, the mechanical connection between the carbon nanotube and the conductive base often is relatively weak and thus tends to be unreliable. Thus, in use, such a carbon nanotube is apt, after a period of time, to break away from the conductive base due to the stress of the electric field force. Such breakage would damage the field emission electron source and/or decrease its performance. Furthermore, in the in-situ synthesis method, control of the growth direction of the carbon nanotube is difficult to achieve during the synthesis process. Thus, the production efficiency thereof is relatively low, and the controllability thereof is less than desired. Still furthermore, the in-situ synthesis method has a relatively high cost.
Additionally, in order for the field emission electron source to successfully emit electrons, the emitter (i.e., the CNT) must have an ability to carry a large current. According to the Fowler-Nordheim equation, the field emitting current of the field emission electron source is determined by the electric field applied thereto and the work function of the emitter. Under the same electric field, the smaller the work function of the emitter (i.e., the lower the energy needed to free an electron therefrom), the bigger the field emitting current of the field emission electron source. However, the work function of carbon nanotubes is 4.55 electron volt. This is merely equal to tungsten. Thus, in use, the work function of a typical carbon nanotube is relatively high. As such, the work function of carbon nanotubes could limit the field emitting current of the field emission electron source.
What is needed, therefore, is a field emission electron source which has a firm mechanical connection and good electrical connection between the carbon nanotube and the conductive base and which thus tends to have satisfactory field emission characteristics and which has an emitter with a low work function.